Curable bonded assemblies capable of being dissociated

ABSTRACT

The invention concerns an adhesive composition for producing thermoset products, capable of being heated by means of an electric field, a magnetic field, an electromagnetic field or an alternating electromagnetic field, and containing filler particles which are metallic, ferromagnetic, ferrimagnetic, superparamagnetic or paramagnetic. Said adhesive composition can be hardened under the action of heat to form a high-resistance stable adhesive assembly, said resulting adhesive assemblies capable of being likewise dissociated under the action of heat.

TECHNICAL FIELD

[0001] The present invention relates to an adhesive compositionaccording to the pre-characterising part of the main claim, whichcontains inductively heatable filler particles, and to its use and aprocess for its curing. The invention also relates to an adhesivecomposite that contains a hardened layer of the adhesive composition, aprocess for the thermal dissociation of the hardened adhesivecomposition, and the use of this process.

PRIOR ART

[0002] Adhesive bonds, i.e. in particular bonded joints, coatings,laminates or cast structural parts are designed so that they can beproduced under mild conditions, are resistant for as long as possibleand have the highest possible strengths. High strengths mean that, inthe case of a repair or recycling, a dissociation of the adhesive bondcan be carried out only under extreme conditions, such as for examplethe action of strong forces or high temperatures. Bonded joints based onhard adhesives are generally dissociable, but are not suitable fortransmitting the high forces necessary for structural bonded joints. Thedissociation of high-strength bonded joints is generally accomplished bythe use of mechanical energy or chemical agents. The latter have thedisadvantage that they cause a high environmental pollution and alsothat the penetration of the agents into the gluelines of structuralbonded joints that are stable over the long term takes far too long.

[0003] DE 43 28 108 A describes the dissociation of floor coverings bymeans of microwave energy. For this purpose a contact adhesive is usedthat is electrically conducting and is filled with copper powder oraluminium powder. These fillers have the disadvantage that the particleshave sizes of a few micrometres and larger. This leads to a non-uniformheating of the contact adhesive.

[0004] DE 199 61 940 A1 describes adhesives for dissociable bondedjoints that contain thermally activatable substances that releasegaseous compounds when they decompose, which then destroy the bondedjoints. This process has the disadvantage that in order to separate thecomposite, the whole structural part or the joined parts and theadhesive have to be heated. This is associated with a high energyexpenditure. Furthermore it is not possible to achieve a locallyrestricted separation of the structural part or of the joined part.

[0005] DE 199 51 599 A1 and DE 199 24 138 A1 describe adhesives fordissociable bonded joints and bonded joints produced therewith, thatcontain externally excitable nanofillers. The dissociation of the bondedjoints is achieved by introducing the joints into an alternatingelectrical, magnetic or electromagnetic field, whereby the nanofillersand the surrounding adhesive are heated. This process has thedisadvantage however that it leads to the heating of the whole adhesive,also at places at which no heating is necessary or desirable, since theexcitable nanofillers are also contained in places in the adhesive orprimer where heating is not necessary in order to achieve the desireddissociation of the bonded joint. Furthermore high temperatures arerequired to separate high-strength bonded joints since chemical bondshave to be broken in order to break down the composite. The describedprocesses furthermore have the disadvantage that a non-specific thermaldecomposition of the adhesive and/or primer occurs when separatinghigh-strength bonded joints. Such processes are therefore unsuitable inparticular for thermosets.

[0006] The production of resistant, high-strength adhesive joints isnormally carried out thermally or photochemically. The conventionalprocesses for producing adhesive bonded joints have the disadvantagehowever that the whole structural part has to be heated in order to curethe adhesive. As a result the process is energy intensive andtime-consuming.

[0007] WO 99/03306 and O. Hahn, A. Kaimann in Adhäsion—Kleben undDichten, October 2001, pp. 35-38 describe a process for the inductivecuring of adhesive joints. In this case adhesives that containinductively activatable fillers are introduced into an electromagneticfield, whereby the inductively activatable fillers are heated and thehardening of the adhesive surrounding the fillers can take place. Theseprocesses have the disadvantage however that the inductively activatablesubstances are not uniformly distributed over the adhesive andaccordingly there is an inhomogeneous heating of the adhesive. As aresult, the strength of such adhesive joints is limited. The processesfurthermore have the disadvantage that a demixing (separation) may occurin the adhesive during the inductive heating process and thedistribution of the thermally activatable substances in the adhesivebecomes even more irregular.

OUTLINE OF THE INVENTION

[0008] The object of the present invention is to overcome thedisadvantages of the prior art and to provide an adhesive compositionthat can be hardened under mild conditions to form a resistant,high-strength adhesive joint. A further object of the present inventionis to provide a process for dissociating such adhesive joints withoutthe long-term resistance of the adhesive joint inevitably sufferingthereby.

[0009] These objects are achieved by the adhesive composition accordingto claims 1 and 11, the process for their curing according to claim 18,the adhesive composite according to claim 19 and the process for thethermal dissociation of the hardened adhesive composition according toclaims 22 and 23. Claims 17 and 24 specify uses of the adhesivecomposition and of the process for dissociating the hardened adhesivecomposition. The subclaims disclose advantageous modifications.

[0010] It was found that adhesive compositions that contain a polymer, apolymer mixture or a reaction resin, as well as particles ofcrosslinking agent, can be heated by means of an electrical field,magnetic field, electromagnetic field, alternating electrical field,alternating magnetic field or alternating electromagnetic field. Theparticles of crosslinking agent consist in this connection of fillerparticles that are metallic, ferromagnetic, ferrimagnetic,superparamagnetic or paramagnetic, as well as crosslinking agent unitsthat are chemically bound to the filler particles.

[0011] A chemical reaction between the crosslinking agent unit and thepolymer or the polymer mixture is triggered by inductive heating of theparticles of crosslinking agent, whereby a polymer network is formed.

[0012] Adhesive compositions within the meaning of the present inventioninclude in particular adhesives, paints, primers, casting compositions,sealants and laminating resins. In particular bonded joints, paintedstructural parts or structural parts provided with a primer, caststructural parts, sealed structural parts or polymer laminates arethereby formed by curing these adhesive compositions. As polymers,polymer mixtures and reaction resins within the meaning of the presentinvention there may be used all polymers, polymer mixtures and reactionresins that are considered suitable for the aforementioned applications.Preferred are crosslinked polymers, particularly preferably polymers orreaction resins from which structural or semi-structural joints can beproduced. In particular epoxide resins, polyurethanes, acrylates, phenolresins, polysulfides or melamine resins are suitable. According to theinvention the filler particles are chemically bound to the crosslinkingagent component. This chemical bonding may be of an ionic, co-ordinativeor covalent nature. Van-der-Waals interactions for example are alsoincluded under this heading.

[0013] In order to produce the chemical bonds according to the inventionit is advantageous if the surfaces of the filler particles aresurface-modified, i.e. if they carry functional groups on their surfacethat can readily react with functional groups of the crosslinking agentunit. The crosslinking agent unit for its part carries at least onefunctional group that on heating can undergo crosslinking reactions withthe polymer, the polymer mixture or the reactive resin. The heating mayin this connection be accomplished either inductively or also in aconventional way. Suitable functional groups that undergo crosslinkingreactions include for example epoxide, amino, thiol, alcohol, acrylate,methacrylate or vinyl groups.

[0014] Chemical groups bound to the crosslinking agent unit that canreact with the filler surface are in particular alkoxysilanes, alkoxytitanates and alkoxy zirconates. These lead to a bonding to the normallymetallic or oxidic surface of the filler particles.

[0015] Filler particles consisting of inductively excitable materials inthe interior of the particles as well as of a particle surfacepredominantly of silicon dioxide are particularly preferred. They may beused in spherical or aggregated form. The predominantly spherical fillerparticles with a core-shell structure (see FIG. 1A, FIG. 2A) can beobtained for example via sol-gel processes or from the reaction ofnanoscale iron oxide with sodium silicate; the aggregated fillerparticles with a silicon dioxide surface (FIG. 1B, FIG. 2B) arepreferably obtained by means of gas phase synthesis. These particles arehereinafter termed composite particles. These composite particlesconsist of aggregates that exhibit the characteristic “sinter neck”;multiple inclusions or domains of the inductively excitable material arefound distributed in the interior of the aggregates, while on thesurface the composite particles consist largely of silicon dioxide.

[0016] Particles with silicon dioxide on the surface have a highlong-term resistance, with respect to moisture, of the bond between thecrosslinking agent unit and the filler particle surface and reactrapidly with the aforementioned chemical groups. Furthermore, thesilicon dioxide layer protects the inductively excitable componentsagainst chemical attack by constituents of the formulation.

[0017] The crosslinking agent particles according to the invention canbe used for example as crosslinking agents for epoxide resins. Thisepoxide resin may for example be a bisphenol A diglycidyl ether hardenedwith a diamine. By using such crosslinking agent particles a thermosetis obtained that can be subjected to high chemical and thermal stress.An individual crosslinking agent particle with epoxide groups is showndiagrammatically by way of example in FIG. 1A. The filler particles mayalso be present in the form of agglomerates, as shown in FIG. 2A. FIG.2A shows diagrammatically a section from an agglomerate of iron oxideparticles in a silicon dioxide matrix.

[0018] The adhesive compositions according to the invention have theadvantage that the distribution of the filler particles is such thatthey are located only at those places where their action is necessary,namely in the crosslinking agent particles. Compared to the previouslyknown adhesive compositions, the filler particles are in this case indirect (i.e. molecular or almost molecular) contact at those places atwhich their effect is to be manifested. In this way a non-specificheating of the whole polymer is avoided during inductive heating. Theadhesive compositions according to the invention may therefore beinductively hardened under mild conditions.

[0019] In addition the adhesive compositions according to the inventionhave the advantage that adhesive joints produced therefrom can also beinductively and cleanly re-separated, with savings in time and energy,without having to add substances that facilitate a separation. Here toothe heating takes place selectively on a molecular level at the site atwhich the bonds are to be broken. This has the advantage that the wholepolymer is not destroyed, as would be the case for example if it wereheated with a welding torch or laser. A non-selective thermaldecomposition of the polymer can thereby be avoided.

[0020] Naturally, a lower induction output, i.e. for example a loweroutput of the high-frequency generator, is necessary for thecrosslinking or curing than for the separation of the adhesive joint.

[0021] According to the invention, in particular adhesive compositionsare suitable that have a content of crosslinking agent particles of 0.1to 80%, preferably 0.5 to 40% and particularly preferably 1% to 30%. Byincreasing the content of crosslinking agent particles or crosslinkingagent units, a higher degree of crosslinking and thus a higher strengthis achieved.

[0022] Advantageously the crosslinking agent particles have an averageprimary particle size, i.e. an average primary particles diameter, ofless than 1000 nm, preferably less than 500 nm and particularlypreferably between 2 nm and 100 nm. A uniform crosslinking of theadhesive composition according to the invention is achieved in this way.Furthermore, such small particle sizes are most suitable from the pointof view of energy economy.

[0023] The filler particles according to the invention may be present inan agglomerated state if the dispersion quality is insufficient. Thecrosslinking agent particles preferably comprise at least threefunctional groups having a crosslinking action, a thermoset therebybeing formed. A thermoset is then generally obtained if three chemicalgroups (polymer systems curing by polycondensation or by polyaddition)or possibly even two chemical groups (polymer systems curing bypolymerisation) are bound to the crosslinking agent. According to theinvention adhesive compositions with crosslinking agent particles thathave, referred to their surface, at least 0.00001 mmole×⁻² functionalgroups having a crosslinking action, are preferred. Typically thedensity of the groups having a crosslinking action is in the range from0.1 to 1 mmole per 100 m² of specific surface of the crosslinking agentparticle. The crosslinking agent particles according to the inventionconsequently form after the curing reaction a stellate crosslinkingcentre in the polymer network. If the functional groups on thecrosslinking agent units are for example epoxide groups, then these maybe employed with BF₃ etherate as curing catalyst for crosslinkingmonofunctional epoxide resins. After curing, a thermoset is then formed;without the crosslinking agent particles according to the invention onlya linear polymer capable of withstanding slight stress would be formed.

[0024] The adhesive compositions according to the invention preferablycontain filler particles that are surface-modified and are selected fromthe group comprising iron, iron alloys and iron-containing metal oxides.Suitable for example are filler particles that are based on iron powder,magnetite powder, superparamagnetic iron oxide or manganese-zinc-ironoxide. For example, nanoscale magnetite powder with a silicon dioxideshell may be surface-modified or functionalised with epoxide groups byreaction with 3-glycidoxypropyltrimethoxysilane. If a sol-gel process isused a reaction with an epoxysilane may at the same time also takeplace, which means that a reaction step can be omitted. However, anotherfunctional silane may also be used for a one-stage or two-stage surfacemodification. The functional groups of the silane are in this connectionchosen so that they can react with the polymer system to be crosslinked.Suitable pairs of functional groups of the crosslinking agent componentand of the polymer system are for example the pairs listed in DE 197 33643 A1, page 4. For example, a thermoset based on isocyanate prepolymersis obtained if the crosslinking agent particles according to theinvention are surface-modified with aminopropyltrimethoxysilane. Byreacting the amino groups with the dimeric isocyanate, urea couplingsare formed as crosslinking sites. In the case of acrylate resins themodification of the crosslinking agent particles is preferably carriedout with silanes containing acrylate or methacrylate groups, and in thecase of mercaptans is preferably carried out with epoxysilanes.

[0025] Particularly preferred are adhesive compositions with fillerparticles that have been produced by flame pyrolysis.

[0026] Such filler particles may be particles or aggregated particleswith superparamagnetic iron oxide domains having a diameter of 3 to 20nm in a silicon dioxide matrix. The production of such particles isdescribed for example by Zachariah et al. in Nanostructured Materials 5,383 (1995) or Ehrman et al. in Journal of Materials Research 14,4551(1999).

[0027] Particularly preferred are the iron oxide-silicon dioxidecomposite particles described in DE 10140089.6 of application date 16Aug. 2001.

[0028] Filler particle domains are understood to be superparamagneticregions spatially separated from one another. On account of the flamepyrolysis process these particles are largely pore-free and contain freehydroxyl groups on the surface. They exhibit superparamagneticproperties when an external magnetic field is applied. The proportion ofsuperparamagnetic iron oxide domains of the filler particles may liebetween 1 and 99.6 wt. %. Regions of superparamagnetic iron oxidedomains spatially separated by the non-magnetic matrix are present inthis range. The range with a proportion of superparamagnetic domainsgreater than 30 wt. %, particularly preferably greater than 50 wt. %, ispreferred. The achievable magnetic effect of the particles according tothe invention also increases in step with the proportion of thesuperparamagnetic regions.

[0029] In these domains the iron oxide may be present in a uniformmodification or in different modifications.

[0030] In addition regions of non-magnetic modifications may also bepresent in the particles. These may be mixed oxides of silicon dioxideand iron oxide. Iron silicalite (FeSiO₄) may be mentioned by way ofexample. These non-magnetic constituents behave like the non-magneticsilicon dioxide matrix as regards superparamagnetism. In other words thedomains remain superparamagnetic, although the saturation magnetisationdrops with increasing proportion of the non-magnetic constituents.

[0031] In addition iron oxide domains may also be present that onaccount of their size do not exhibit superparamagnetism and induce aremanence. This leads to an increase in the volume-specific saturationmagnetisation. Suitably adapted composite particles can be produceddepending on the field of application.

[0032] A particularly preferred superparamagnetic iron oxide domain isgamma-Fe₂O₃ (γ-Fe₂O₃), Fe₃O₄, and mixtures thereof.

[0033] Apart from the spatial separation of the superparamagnetic ironoxide domains, the silicon dioxide matrix also has the task ofstabilising the oxidation state of the domains. Thus, for example,magnetite is stabilised as superparamagnetic iron oxide phase by asilicon dioxide matrix.

[0034] According to a particular embodiment the carbon content of thefiller particles may be less than 500 ppm. Particularly preferably thecontent is less than 100 ppm.

[0035] The filler particles may furthermore have a chloride content of50 to 1000 ppm, originating from the production of the particles. Theparticles are obtained by a flame pyrolysis process in whichchlorine-containing precursors are reacted for example in ahydrogen/oxygen flame. The particles that are formed may containchlorine for example in the form of oxychlorides from the reaction thathas not gone to completion, as well as chlorine in the form ofhydrochloric acid. If these compounds are enclosed in the particles thatare formed, the chloride content of the particles cannot be reducedfurther even by purification steps, without the particles beingdestroyed. The chloride content can be reduced to values of ca. 50 ppmby purification steps.

[0036] The filler particles may have different degrees of aggregationdepending on the conditions of the flame pyrolysis process. Influencingparameters may include the residence time, temperature, pressure, thepartial pressures of the compounds that are used, and the nature andsite of the cooling step after the reaction. A broad spectrum rangingfrom largely spherical to largely aggregated composite particles maythus be obtained.

[0037] The BET surface, determined according to DIN 66131, of the fillerparticles may vary over a wide range from 10 to 600 m²/g. Particularlypreferably the range is between 50 and 300 m²/g.

[0038] In a preferred embodiment of the composite particles the“blocking temperature”, i.e. the temperature below whichsuperparamagnetic behaviour can no longer be detected, may be no morethan 150 K. This temperature may depend on, apart from the compositionof the particle, also on the size of the superparamagnetic domains andtheir anisotropy.

[0039] The composite particles of superparamagnetic iron oxide andsilicon dioxide as filler particles are then reacted on the surface withfor example silanes, which may additionally carry groups that can reactwith the adhesive. In this way the inductively excitable crosslinkingagent particles are obtained.

[0040] In an advantageous modification of the invention the fillerparticles are bound via a thermally labile group to the crosslinkingagent units. Suitable thermally labile groups are in particular azogroups, carbonate groups or ethylene groups with sterically demandingsubstituents. When adhesive compositions with such thermally labilegroups are heated a bond rupture occurs at a specific temperature,whereby for example nitrogen or carbon dioxide are formed in theexamples given above, or the carbon-carbon bond of the ethylene groupwith sterically demanding substituents is ruptured. A filler particle towhich the crosslinking agent units are bound via a thermally labilegroup is shown for example in FIG. 1B or FIG. 2B, in which the fillerparticles are present in agglomerated form.

[0041] In a variant the adhesive composition, which can be heated bymeans of an electrical field, magnetic field, electromagnetic field,alternating electrical field, alternating magnetic field or alternatingelectromagnetic field, contains a polymer, a polymer mixture or areactive resin, a thermally labile substance, a crosslinking agentcomponent, as well as filler particles that are metallic, ferromagnetic,ferrimagnetic, superparamagnetic or paramagnetic.

[0042] Such adhesive compositions are particularly suitable for theproduction of hardened adhesive joints that are to be re-dissociated byinductive heating.

[0043] Adhesive compositions within the meaning of this variant alsoinclude in particular adhesives, paints, primers, casting compositions,sealants and laminating resins. All polymers and polymer mixtures thatare suitable for the uses mentioned in the introduction may also beregarded as polymers, polymer mixtures and reactive resins within themeaning of this variant. Crosslinked polymers are also preferred forthis variant, particularly preferably polymers or reactive resins fromwhich structural or semi-structural joints can be produced. Inparticular epoxide resins, polyurethanes, acrylates, phenol resins,polysulfides or melamine resins are suitable.

[0044] The inductively excitable fillers are present either in the formof singular nanoparticles or aggregates, or in. the form ofagglomerates. They are preferably found in direct contact with thethermally labile substance, so that the heating takes place selectivelyat the site at which bonds are to be ruptured. Particularly preferablythe thermally labile substance is therefore bound to the inductivelyexcitable fillers. Also preferred are filler particles that carry asilicon dioxide-containing coating. These have a high long-termresistance to moisture.

[0045] The crosslinking agent carries functional groups that undergocrosslinking reactions with the polymer, the polymer mixture or thereactive resin, which may be thermally accomplished either inductivelyor also in a conventional way. Suitable groups that undergo crosslinkingreactions include for example epoxide, amino, thiol, alcohol, acrylate,methacrylate or vinyl groups.

[0046] The adhesive compositions of this variant accordingly have theadvantage that, after they are cured, adhesive joints are formed thatcan be cleanly separated by means of inductive heating, with savings intime and energy. If the filler particles are in direct contact with thethermally labile substance, then the heating is carried out selectivelyat the site at which the bonds of the thermally labile substance are tobe ruptured. This has the advantage that the whole polymer is notdestroyed, as would be the case for example if the heating were carriedout with a welding torch or laser. A non-selective thermal destructionof the polymer may therefore be avoided.

[0047] As thermally labile substances added to the adhesive composition,in particular those substances are suitable that have average particlesizes between 2 nm and 100 μm, preferably between 2 nm and 1 μm andparticularly preferably between 2 nm and 200 nm. Such particles may bepresent individually or in the form of agglomerates. If adhesive jointsthat are obtainable by curing such adhesive compositions are heated,then bonds are ruptured or phase transformations take place in thesethermally labile substances and the polymer network is destabilised,whereby a rupture of the adhesive joint is possible.

[0048] In an advantageous modification the thermally labile substance isa blowing agent that forms a gas under the action of heat, the gasformation temperature being higher than the temperature at which thecrosslinking of the adhesive composition starts.

[0049] The dissociation of adhesive composites that have been producedfrom such adhesive compositions is carried out by inductive heating ofthe particles coupled to the blowing agent, via the decompositiontemperature of the blowing agent or of the blowing agent component andthe resultant thermal decomposition. The gaseous decomposition productsthat are formed “blast” the adhesive composite apart. Suitable blowingagents include for example substances that split off water ofcrystallisation (e.g. aluminium nitrate), organic carboxylic acids (e.g.oxalic acid, glutaric acid), azo compounds (e.g. azodicarbonamide,azoisobutyronitrile), or fluorinated hydrocarbons.

[0050] Particularly preferred blowing agents are azodicarbonamide andsulfohydrazides, such as in particular toluene sulfohydrazide andoxygen-bis(benzosulfohydrazide). In general derivatives ofazodicarbonamide are also suitable. The blowing agents may optionally beactivated with zinc salts. These blowing agents have the advantage thatthey release a large amount of gas, are sparingly soluble in organicmedia, decompose at 180° to 200° C., i.e. above the temperature of useof normal bonded joints, and are not toxic. If necessary thedecomposition temperature may be reduced by additional activators andthereby adjusted to the conditions under which it is intended todissociate the adhesive joints. The poor solubility of theaforementioned blowing agents in organic media has the result that theblowing agent is not dissolved in the adhesive and consequently theintimate contact between the inductively heatable filler particles andthe blowing agent in the adhesive is retained. In this way theeffectiveness of the resin formulation according to the invention isfurther enhanced.

[0051] In a further advantageous modification of the adhesivecomposition according to the invention collective particles are first ofall formed from blowing agent and filler particles, in which the blowingagent may optionally be bound to the filler particles. Collectiveparticles are therefore understood to denote particles that contain boththe blowing agent as well as the filler particles. These collectiveparticles are obtained by precipitation, compression, microencapsulationor binding of blowing agents and filler particles with a polymer. Thesize of such collective particles is limited simply by the subsequentuse. Such collective particles have the advantage that inductivelyheatable blowing agent particles that are readily compatible with theadhesive matrix can be produced. The blowing agent cannot therefore bereleased by the resin system and the intimate contact betweeninductively heatable filler particles and the blowing agent is retained.

[0052] Preferably the polymer for the formation of the collectiveparticles is expandable polystyrene. The collective particles areaccordingly polystyrene beads that contain at the same time theinductively heatable filler particles and a blowing agent that isnormally used to expand polystyrene. Such polystyrene beads preferablyhave a size of 1 μm to 1 mm. When adhesive composites that have beenproduced from adhesive compositions that contain these polystyrene beadsare inductively heated, this leads to an expansion of the polystyreneparticles and thus to the dissociation of the adhesive composite.

[0053] The adhesive composition according to the invention is preferablyused for adhesives, paints, sealants, primers, matrix resins or castingresins.

[0054] A process for curing the adhesive composition according to theinvention to form adhesive joints consists in inductively heating theadhesive composition by means of an electrical field, magnetic field,electromagnetic field, alternating electrical field, alternatingmagnetic field or alternating electromagnetic field, to a temperature atwhich the crosslinking of the adhesive composition starts. This processhas the advantage that the duration of the inductive heating is normallyin the range from seconds to minutes, and is thus significantly shorterthan in the conventional thermal curing processes. Accordingly it isparticularly suitable for curing the adhesive compositions according tothe invention on sensitive objects. Also, this process is veryenergy-efficient.

[0055] The curing process according to the invention may in particularbe carried out as described in Ortwin Hahn, Andrea Kaimann,Adhäsion—Kleben und Dichten, October 2001, pp. 35 to 38 for adhesivecompositions containing coarse particulate fillers. The addition ofcuring catalysts and activators, as is also described in unpublishedspecification DE 10127704.0, is advantageous.

[0056] The adhesive composite obtained by the process according to theinvention for curing the adhesive composition according to the inventioncontains at least one hardened adhesive layer. In particular such anadhesive composite may be an adhesive joint, a cast structural part, asealing structural part or a polymer laminate. The cured adhesive layermay be a paint layer or a primer layer.

[0057] The process according to the invention for the thermaldissociation of the adhesive composite that can be obtained by curingthe adhesive composition according to the invention is carried out byinductively heating the hardened layer of the adhesive composition bymeans of an electrical field, magnetic field, electromagnetic field,alternating electrical field, alternating magnetic field or alternatingelectromagnetic field. If the adhesive composition contains thecrosslinking agent particles according to the invention, then thehardened layer of the adhesive composition is heated in the processaccording to the invention to a temperature that lies above the ceilingtemperature of the crosslinking points. If the adhesive compositioncontains filler particles and a thermally labile substance, then thehardened layer of the adhesive composition is heated inductively to atemperature at which the thermally labile bonds of the thermally labilesubstance or of the thermally labile group rupture.

[0058] In the process according to the invention for the thermaldissociation of the hardened adhesive composites, the filler particlesare thus inductively heated, whereby a chemical reaction is initiated inwhich either the thermally labile substances effect a rupture of thecrosslinking points of the polymer network due to bond rupture,formation of gas and/or swelling effects, or in which, due to theinductive heating of the filler particles, a bond rupture takes place atthe crosslinking points lying in the immediate vicinity of the fillerparticles.

[0059] The dissociation of the adhesive composites according to theinvention thus takes place selectively by the action of thehigh-frequency energy from a conventional induction coil. Due toresultant eddy currents, particle movements in the alternating field andhystoresis losses, the metallic, ferromagnetic, ferrimagnetic,superparamagnetic or paramagnetic filler particles according to theinvention present in the polymer network are heated. At the same timethere is also a heating of the polymer system in the immediateenvironment of the filler particles. The induction voltage is chosen sothat the heat that is generated is sufficient to dissociate thecrosslinking points in the polymer network and to destroy the latterthermally in the case where heating is combined with blowing agents. Theinduction frequencies are preferably between a few kilohertz and about35 megahertz. The equipment, parameters and equipment adjustmentsrequired in each case depend on the filler that is used and its contentin the polymer system. In particular, the particle size distribution,Curie temperature, permeability, electrical resistance, coefficient ofthermal expansion and the specific thermal capacity are quantities onwhich depends the achievable temperature for a specific setting of theequipment. The temperature required for the dissociation depends on thethermal stability of the respective polymer system and blowing agent. Ifthe filler particles that are used are chemically bound to thecrosslinking sites of the polymer system, then the filler itself is aconstituent of the polymer system. On account of the localised vicinityof the chemical bonds to be separated to the filler particles introducedaccording to the invention and heatable by induction, the separation isin this case particularly effective.

[0060] The process according to the invention for dissociating adhesivecomposites may be used on bonded joints that consist only of theadhesive itself according to the invention, though there may also beused an adhesive primer based on the adhesive compositions according tothe invention in combination with a commercially available adhesive. Inthis case the dissociation of the composite takes place selectively inthe primer layer. The adhesive remains on one of the two bonded parts.

[0061] If the adhesive composition according to the invention is a paintthat is to be inductively pickled, then in particular adhesivecompositions according to the invention are advantageous in which thefiller particles are bound to the crosslinking agent component. This hasthe advantage that no chemicals, no expensive equipment and no highlabour expenditure are necessary for the pickling. Furthermore thepaints that can be inductively pickled are particularly suitable forsensitive substrates. As examples there may be mentionedfibre-reinforced composite plastics, in which the base polymers aredamaged by the pickling chemicals, or the fibres may be exposed ordamaged during the grinding. Both phenomena lead to an unallowableweakening of the sensitive structural part.

[0062] The process according to the invention for the inductive picklingis suitable in particular for carbon fibre-reinforced structural partsof aircraft or glass fibre-reinforced composites in ships' carcasses andwind power blades.

EXAMPLES OF USE

[0063] Without restricting its general applicability, the adhesivecomposition according to the invention, the process for curing thelatter to form an adhesive composite and the dissociation of thisadhesive composite are described in more detail hereinafter with the aidof examples of use.

Example 1 Curing and Dissociation of a Bonded Joint with InductivelyExcitable Filler Particles Bound to a Crosslinking Agent

[0064] 1a) Nanoscale Magnetite Coated with Silicon Dioxide

[0065] 43.3 g of iron(III) chloride hexahydrate are dissolved in 370 mlof water and freed from dissolved oxygen by passing nitrogen through thesolution. 15.9 g of iron(II) chloride tetrahydrate are added and asolution of 25.6 g of sodium hydroxide in 100 ml of water is addeddropwise within 2 hours, with stirring, with a precision glass stirrerunder a flowing stream of nitrogen. A finely particulate blackprecipitate of Fe₃O₄ is thereby formed. A solution of 22 g of Na₂Si₃O₇(annealing loss 17 wt. %) in 80 ml of hot water is then added dropwisewithin 30 minutes. The silicic acid is now precipitated, with furtherstirring, by slow dropwise addition of hydrochloric acid (14 ml of 37%hydrochloric acid made up to 50 ml with water). The precipitate isfiltered and made into a slurry five times with distilled water and isin each case refiltered in order to separate the sodium chloride formed.The resulting material consists of agglomerated nanoparticles. Theprimary particles have a diameter of about 8 nm (determined bytransmission electron microscopy) and the agglomerates have a diameterof about 400 nm (determined by light scattering).

[0066] 1b) Modification of the Nanoscale, Inductively Excitable FillerParticles

[0067] 20 g of the nanoscale magnetite produced according to Example 1awith a residual moisture content of 40% or a comparable material fromanother source is made into a slurry with acetone, made up to a totalvolume of 300 ml, acidified with 0.3 ml of conc. hydrochloric acid,following which 15 g of epoxycyclohexyltrimethoxysilane are added. Thewhole is stirred for 24 hours with a precision glass stirrer and thendried in vacuo on a rotary evaporator. The surface-modified fillercarries cycloaliphatic epoxide groups on the surface and is able to actas a crosslinking agent in the adhesive system.

[0068] 1c) Incorporation of the Surface-Modified Filler into an Adhesive

[0069] 10 g of3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexane-carboxylate, 10 g ofcyclohexene oxide, 0.2 g of (tolylcumyl) iodoniumtetrakis(pentafluorophenyl) borate and 0.2 g of ascorbicacid-6-hexadecanoate are stirred until a homogeneous mixture is formed(base mixture according to unpublished specification DE 10127704.0). 4 gof the surface-modified filler according to b) are incorporated using adissolver. After stirring for 15 minutes a homogeneous thixotropic masshas formed, which is used hereinafter as adhesive.

[0070] 1d) Bonding with Inductive Curing and Re-Dissociation of theBonded Joint

[0071] 25 mm-wide and 4 mm-thick parts to be joined consisting of glassfibre-reinforced polyester are bonded with the adhesive producedaccording to c). For this, a part to be joined is coated in the joiningregion with a 0.2 mm-thick adhesive layer, the second part is placedthereon, and both parts are fixed under a light pressure (ca. 0.02N/mm²). The subsequent curing of the adhesive is carried out byexcitation with an M230 semiconductor generator from STS. The excitationfrequency of this generator is 300 kHz. A coil with three windings andan internal diameter of 3 cm is used for the inductive excitation of theadhesive in the bonded joint. The adhesive surface is aligned in themiddle of the coil, perpendicular to the coil axis. The adhesive iscured at an output of 1000 W and an action time of 5 minutes, a firmjoint thereby being obtained. This is then dissociated within 60 secs byincreasing the generator output to 3000 W. In a comparison examplewithout inductively excitable nanoscale filler particles, it is notpossible to cure the adhesive even at an output of 3000 W and an actiontime of 10 minutes.

Example 2 Bonded Joint that Can be Dissociated by an InductivelyExcitable Blowing Agent

[0072] 2a) Formulation from Magnetite Powder and Blowing Agent

[0073] 20 g of the material produced according to la) (residual moistureof the filter cake 40%) or a nanoscale magnetite powder obtained fromanother source is suspended in 100 ml of ethanol and 20 g ofoxy-bis(benzosulfohydrazide) are added as blowing agent. The mixture isheated for 4 hours at 70° C. with stirring, and the solvent is thenremoved on a rotary evaporator. The dry formulation is ground in a ballmill for 5 minutes and then screened. The fraction with a grain size ofnominally less than 63 μm is used for the further tests.

[0074] 2b) Incorporation of the Formulation from Example 2a) in anAdhesive

[0075] 8 g of3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexane-carboxylate, 2 g ofpoly(tetrahydrofuran) of M_(n)=250, 0.1 g of (tolylcumyl) iodoniumtetrakis (pentafluorophenyl) borate and 0.04 g of ascorbicacid-6-hexadecanoate are stirred until the components have dissolved inone another (base mixture according to unpublished specification DE10127704.0). 2 g of the formulation are then stirred in, whereupon thematerial assumes a doughy consistency. A part of the sample cures within30 minutes at 90° C. to form a tack-free polymer.

[0076] 2c) Bonding and Re-Dissociation of a Plastics Joint

[0077] 25 mm-wide and 3 mm-thick polypropylene parts to be joined arepretreated by fluorination according to the prior art, coated with a ca.0.2 mm-thick layer of the inductively excitable formulation, followingwhich a second part is placed thereon and is cured under a lightpressure (ca. 0.02 N/mm²) for 30 minutes in an oven at 90° C. Thesubsequent separation of the joint is carried out by excitation with anM230 semiconductor generator from STS. The excitation frequency of thisgenerator is 300 kHz. A coil with three windings and an internaldiameter of 3 cm is used for the inductive excitation of the adhesive inthe bond joint. The adhesive surface is aligned in the middle of thecoil, perpendicular to the coil axis. With an output of 1500 W and anaction time of 25 secs. the blowing agent has decomposed and therebydissociated the bonded joint. In a comparison example without thenanoscale magnetite powder, the bonded joint cannot be dissoicated evenunder the action of an output of 3000 W for 2 minutes.

[0078] The analytical data of the iron oxide-silicon dioxide compositeparticles used in Examples 3 to 6 are shown in Table 1. The productionof these particles is described in DE 10140089.6. TABLE 1 Analyticaldata of the iron oxide-silicon dioxide composite particles of Examples3, 4, 5 and 6 Use in Example 3, 4, 5 6 Iron oxide⁽*⁾ wt. % 50 50 Carboncontent ppm 70 <10 Chloride content ppm 368 635 Saturation magnetisationAm²/kg 17 12.5 gamma-Fe₂O₃ crystallite size nm 10.8 15.1 Blockingtemperature (ca.) K 100 n.d. BET surface m²/g 146 88

Example 3 Dissociation of a Glass Bond Based on an Adhesive withInductively Excitable Blowing Agent

[0079] 3a) Formulation from Composite Particles Produced by FlamePyrolysis and a Binder

[0080] 25 g of nanoscale composite particles produced by flame pyrolysisand consisting of silicon dioxide and iron oxide having the propertiesshown in Table 1 are suspended in 100 ml of ethanol and 20 g ofoxy-bis(benzosulfohydrazide) are added as blowing agent. The mixture isheated for 5 hours at 60° C. with stirring, and the solvent is thenremoved on a rotary evaporator. The dry formulation is ground in a ballmill for 3 minutes and then screened. The fraction with a grain size ofnominally less than 63 μm is used for the further tests.

[0081] 3b) Incorporation of the Formulation from Example 3a) in anAdhesive, and Adhesive Tests

[0082] 300 g of the moisture-hardening one-component polyurethaneadhesive Dinitrol PUR 501 FC (Dinol GmbH) are modified in a Planimax(Molteni) mixer provided with kneading hooks, with 10 g of theformulation produced according to Example 3a and consisting of blowingagent and inductively excitable nanofiller. The mixture is kneaded for15 minutes at setting 1 (150 rpm) in a dry atmosphere.

[0083] A thick layer bond between a sand-blasted and degreased aluminiumsheet and a 3 mm-thick float glass panel is prepared using the therebymodified adhesive. The overlap length is 25 mm and the adhesive layerthickness is 3 mm.

[0084] After a curing time of 1 week at 25° C. and 50% relativeatmospheric humidity the joint is re-separated by inductive excitation.The separation of the joint is carried out by excitation with an M230semiconductor generator from STS. The excitation frequency of thisgenerator is 300 kHz. A coil with three windings and an internaldiameter of 3 cm is used for the inductive excitation of the adhesive inthe bonded joint. The adhesive surface is aligned in the middle of thecoil, perpendicular to the coil axis. At an output of 3000 W and anaction time of 2 minutes the adhesive is destroyed by the expansion ofthe blowing agent. The two joined parts can easily be separated from oneanother.

Example 4 Dissociable Bonded Joint Based on an Inductively DissociableAdhesive Primer

[0085] 5 g of the formulation prepared according to Example 3a andconsisting of blowing agent and inductively excitable nanofiller arestirred into 200 g of the Sika-Primer 206G+P (Sika AG) adhesive primer.The primer is applied to a sand-blasted and degreased aluminium sheet soas to cover the latter. After an aeration time of 1 hour the sheetpretreated in this way is bonded with a 3 mm-thick layer of beechplywood. Sikaflex 254 (Sika AG) is applied as adhesive in a thickness of3 mm, and the overlap length is 25 mm. The adhesive hardens within 1week at a relative atmospheric humidity of 50% and at 25° C. The jointis then re-separated by inductive excitation. The excitation of theadhesive for the separation is carried out with an M230 semiconductorgenerator from STS. The excitation frequency of this generator is 300kHz. A coil with three windings and an internal diameter of 3 cm is usedfor the inductive excitation of the adhesive in the bonded joint. Theadhesive surface is aligned in the middle of the coil, perpendicular tothe coil axis.

[0086] At an output of 3000 W and an action time of 2 minutes theadhesive primer applied to the aluminium side is destroyed by theexpansion of the blowing agent. The parts of the joint can easily beseparated from one another, the adhesive remaining selectively on theplywood.

Example 5 Curing an Elastic Adhesive and Testing the Adhesive Properties

[0087] The elastic adhesive Elastosol M83 (Tivoli, Hamburg) is aone-component, heat-curing metal adhesive based on polybutadiene. On thebasis of a comparison of the curing in a conventional oven with theinductive curing according to the invention, it will be shown thatequivalent bonding results are achieved with both types of curing. Inthis connection the inductive curing occurs more rapidly.

[0088] 25 g of the iron oxide-silicon dioxide composite particlesaccording to Table 1 are incorporated in 250 g of the adhesive ElastosolM83 using a Planimax mixer (Molteni) equipped with kneading hooks.Kneading is first of all carried out for 5 minutes at 150 revolutionsper minute and then for a further 30 minutes at 450 rpm (setting 3). Themixture is then kneaded for a further 5 minutes in vacuo to degass it(setting 3). The material according to the invention that is thusobtained is used for the adhesive tests.

[0089] In order to check the adhesive properties, tensile/shear samplesare produced on the basis of DIN EN 1465, one part to be joinedconsisting of 1.15 mm-thick rolled aluminium sheet material(AlMg_(0.4)Si₁₂) and another part to be joined consisting of 4 mm-thick,glass fibre-reinforced polypropylene. The aluminium is ground anddegreased with butanone. The polypropylene is pretreated in alow-pressure plasma with air as working gas.

[0090] Adhesive samples are first of all prepared with the modifiedadhesive according to the invention and inductively hardened. The curingof the adhesive is carried out by excitation with an M230 semiconductorgenerator from STS. A water-cooled flat coil with three windings and adiameter of 8 cm is used for the inductive excitation of the adhesive inthe bonded joint. The coil is placed on the joined part of polypropyleneand the adhesive is cured at an output of 1000 W and an action time of10 minutes The samples have a tensile-shear strength of 10.4±0.6 MPa,with a cohesive fracture of the adhesive.

[0091] For purposes of comparison, tensile/shear samples are preparedwith the unmodified adhesive. In this case the adhesive is cured in aconventional manner according to the manufacturer's instructions. Thecuring is carried out over 30 minutes at 180° C. in an oven. Thetensile-shear strength of these samples is 10.1±0.4 MPa.

Example 6 Modification of a Melt Adhesive for the Selective Dissociationof Bonded Joints

[0092] The melt adhesive B40166 (Heinrich Bühnen GmbH) is modified with7 wt. % of iron oxide-silicon dioxide composite particles according toTable 1 in a Brabender double-screw extruder at a screw speed of 60 rpmand a screw temperature of 220° C. in all heating zones. The meltadhesive is granulated and applied with an HB 500 application device to5 mm-thick beech plywood. A second piece of beech plywood is immediatelypressed onto the melt adhesive. The joint is firm within 1 minute. Thedissociation of the bonded joint is carried out inductively with theM230 semiconductor generator from STS and a water-cooled flat coil withthree windings and a diameter of 8 cm. The coil is placed on the plywoodand an output of 3000 W is adjusted at the semiconductor generator.After an action time of 60 seconds the two plywood plates can be takenapart and, after renewed inductive heating, can be rebonded to oneanother.

1. Adhesive composition for the production of thermosets, thecomposition being able to be heated by means of an electrical field,magnetic field, electromagnetic field, alternating electrical field,alternating magnetic field or alternating electromagnetic field andcontaining a polymer, a polymer mixture or a reaction resin, andcrosslinking agent particles, wherein the crosslinking agent particlescomprise filler particles that are ferromagnetic, ferrimagnetic,superparamagnetic or paramagnetic, and crosslinking agent unitschemically bound to these filler particles.
 2. Adhesive compositionaccording to claim 1, wherein the content of crosslinking agentparticles is 0.1 wt. % to 80 wt. %.
 3. Adhesive composition according toclaim 1, wherein the crosslinking agent particles have average primaryparticle sizes between 2 nm and 1000 nm.
 4. Adhesive compositionaccording to claim 1, wherein the crosslinking agent particles have,with respect to their surface, at least 0.00001 mmole×m⁻² of functionalgroups with a crosslinking action.
 5. Adhesive composition according toclaim 1, wherein the filler particles are surface-modified.
 6. Adhesivecomposition according to claim 1, wherein the filler particles areselected from the group consisting of iron, iron alloys, iron-containingmetal oxides, and mixtures thereof.
 7. Adhesive composition according toclaim 5, wherein the filler particles have a core-shell structure andare produced by sol-gel processes or from the reaction of nanoscale ironoxide with sodium silicate.
 8. Adhesive composition according to claim5, wherein the filler particles are superparamagnetic iron oxide-silicondioxide composite particles produced by flame pyrolysis.
 9. Adhesivecomposition according to claim 1, wherein the crosslinking agent unitsare bound via a thermally labile group to the filler particles. 10.Adhesive composition according to claim 9, wherein the thermally labilegroup is an azo group, a carbonate group or an ethylene group withsterically demanding substituents.
 11. Adhesive composition for theproduction of thermosets according to claim 1, wherein said adhesivefurther comprises a thermally labile substance.
 12. Adhesive compositionaccording to claim 11, wherein the thermally labile substance hasaverage particle size between 2 nm and 100 μm.
 13. Adhesive compositionaccording to claim 11, wherein the thermally labile substance is ablowing agent that forms gas under the action of heat, in which the gasformation temperature is higher than the temperature at which thecrosslinking of the adhesive composition starts.
 14. Adhesivecomposition according to claim 13, wherein the blowing agent is selectedfrom the group consisting of an azodicarbonamide and a sulfohydrazide,and mixtures thereof.
 15. Adhesive composition according to claim 13,wherein collective particles are included that contain the blowing agentand the filler particles and are obtainable by precipitation,compression, microencapsulation or bonding of blowing agent and fillerparticles with a polymer.
 16. Adhesive composition according to claim15, wherein the polymer for the formation of the collective particles isexpandable polystyrene.
 17. Use of the adhesive composition according toclaim 1, for adhesives, paints, sealants, primers, matrix resins orcasting resins.
 18. Process for the curing of an adhesive compositionaccording to claim 1, wherein the adhesive composition is inductivelyheated by means of an electrical field, magnetic field, electromagneticfield, alternating electrical field, alternating magnetic field oralternating electromagnetic field to a temperature at which thecrosslinking of the adhesive composition starts.
 19. Adhesive compositethat comprises at least one adhesive layer that is obtainable by curingthe adhesive composition according to claim
 1. 20. Adhesive compositeaccording to claim 19, wherein the at least one adhesive layer is apaint layer or primer layer.
 21. Adhesive composite according to claim19 that is a bonded joint, a cast structural part, a sealed structuralpart or a polymer laminate.
 22. Process for the thermal dissociation ofan adhesive composite that can be obtained by curing the adhesivecomposition according to claim 1, wherein the hardened layer of theadhesive composition is heated by means of an electrical field, magneticfield, electromagnetic field, alternating electrical field, alternatingmagnetic field or alternating electromagnetic field to a temperaturethat lies above the ceiling temperature of the crosslinking points. 23.Process for the thermal dissociation of an adhesive composite that canbe obtained by curing the adhesive composition according to claim 1,wherein the hardened layer of the adhesive composition is heated bymeans of an electrical field, magnetic field, electromagnetic field,alternating electrical field, alternating magnetic field or alternatingelectromagnetic field to a temperature at which the thermally labilebonds of a thermally labile substance and/or thermally labile grouprupture.
 24. Use of the process according to claim 22 for picklingpaints.
 25. Use according to claim 24 in aircraft construction andshipbuilding.